The present invention relates to hydraulic drive systems of the type including a variable displacement hydrostatic pump or motor unit, and more particularly, to such a drive system of the type including a pump-motor unit which operates as a pump during a portion of the vehicle operating cycle, and as a motor during another portion of the vehicle operating cycle. Even more particularly, the present invention relates to such a hydraulic drive system in which the pump-motor unit is of the type having a variable swashplate, the direction of displacement and tilt angle of which are varied only by changes in a control pressure communicated to a pair of fluid pressure actuators.
Although the hydraulic drive system of the present invention may be utilized in hydraulic drive systems of various types, including such drive systems which effectively serve as the primary vehicle transmission during at least most of the vehicle operating cycle, the present invention is especially advantageous when used on a hydraulic drive system which comprises part of a vehicle hydraulic regenerative braking system, and will be described in connection therewith. However, it should be understood that the present invention is not limited to use only in connection with a vehicle drive system, but could be used with various types of stationary and/or industrial equipment. Therefore, as used hereinafter and in the appended claims, references to receiving torque from a “drive line” or transmitting drive torque to such a “drive line”, will be understood to mean and include any sort of torque transmitting drive line, whether or not such drive line is part of a vehicle drive system, or part of some other, non-vehicle type of drive system.
In a vehicle hydraulic drive system of the type having regenerative braking capability, the drive system includes, in addition to the pump-motor unit referenced previously, a high pressure accumulator and a low pressure accumulator, and the appropriate control valving to control the flow of fluid, especially between the high pressure accumulator and the pump-motor unit. In the hydraulic drive system of the present invention, the swashplate of the pump-motor unit is tilted in a first displacement orientation when the pump-motor unit is operating in the pumping mode (when the pump-motor unit is receiving torque from the drive line) and then the swashplate moves “over-center” and is displaced in a second displacement orientation when the pump-motor unit is operating in the motoring mode (when the pump-motor unit is transmitting torque to the drive line).
Furthermore, as is now well known to those skilled in the art of such drive systems, the “control pressure” used to drive the swashplate of the pump-motor unit toward either its first displacement orientation or its second displacement orientation is typically provided by the high pressure accumulator. Disposed between the high pressure accumulator and the pump-motor unit is a main control valve means which would communicate control pressure to one of the fluid pressure actuators of the swashplate, while communicating the other fluid pressure actuator of the swashplate to the low pressure side of the system. Such an arrangement differs from the typical variable displacement axial piston pump which receives its control pressure from a charge pump driven off of the same input shaft which drives the main axial piston pump rotating group (cylinder barrel and pistons).
Another difference between the pump-motor unit of the type with which the present invention is utilized and the conventional, variable displacement axial piston pump relates to the centering of the fluid pressure actuators and the swashplate. In the conventional axial piston pump, each of the fluid pressure actuators is typically spring-centered, but in the pump-motor unit of the invention, the actuators would most likely not include centering or return springs. There are several reasons for omitting the conventional centering springs from the pump-motor unit of the invention. The conventional centering springs are large (thus complicating the packaging) and are expensive, and require a substantial amount of hydraulic energy to overcome, in order to move the swashplate from the neutral position to a displaced position. Finally, the presence of large, high force centering springs in the pump-motor unit of the invention would substantially increase the response time for the unit to transition between its various modes.
Therefore, the displacement of the swashplate is a function of (i.e., proportional to) substantially only the pressure differential between the high side of the system (in the subject embodiment, the high pressure accumulator) and the low pressure side of the system (such as a low pressure accumulator). It will be understood by those skilled in the art that, although reference has been made herein to the low pressure side of the system comprising a low pressure accumulator, such is not an essential feature of the invention. All that is essential is that there be a “low pressure source”, which could be a low pressure accumulator, but could also merely be the case drain region of the pump-motor unit, or a reservoir (whether or not pressurized). All that is essential is that there be a source of pressure at least slightly greater than atmospheric pressure, for reasons which will become apparent subsequently.
Although the hydraulic drive system of the type described above has proven to be very satisfactory in operation, it has been observed during the development of a commercial embodiment of the system that, in the absence of the present invention, there is at least one operating condition under which the pump-motor unit may not operate in the manner desired. In the event of leakage from the high pressure side of the system (or if there is an extended “idle” period), thus permitting the high pressure accumulator to leak down and be at the same pressure as the low pressure accumulator, then both of the fluid pressure actuators of the pump-motor unit would be subjected to the same, low pressure. Under the condition just described, the pump swashplate would move to a centered (neutral, zero displacement) position. When the system logic would subsequently command displacement of the swashplate, so that the pump-motor unit would operate in either the pumping mode or the motoring mode, there would be no pressure differential to move the swashplate from the neutral position toward the appropriate, displaced position. Thus, the pump-motor unit, in the absence of the invention, would thereafter be unable either to convert input torque from the drive line into stored pressure in the high pressure accumulator, if that is what was commanded, or to transmit stored pressure into torque to be transmitted to the drive line, if that is what was commanded.